“Syn-Effect” in the Conversion of (E)-Vinylic Sulfones to the Corresponding Allylic Sulfones

It was found that (E)-vinylic sulfones preferentially afford (Z)-allylic sulfones as kinetically-controlled products by treatment with a base under mild conditions, while (Z)-vinylic sulfones give (E)-allylic sulfones. Such stereochemical relationship was rationalized by “syn-effect”, and its relative degree for various substituents was determined by observation of E/Z ratios of the allylic sulfones resulted from the corresponding γ-mono- or γ,γ-disubstituted vinylic sulfones as follows: RO– (R = CH3, C2H5) ArO– (Ar = p-CH3OC6H4, p-CH3C6H4, C6H5, p-NO2C6H4) ≥ AcO– > Cl– ≥ Br– > CH3– > CH3S– ≥ –CH2– (cyclic and acyclic) > (CH3)2CH– >> (CH3)3C–, C6H5–. X-Ray crystallography was performed for some vinylic sulfones to reveal the origin of the “syn-effect”.